The International Association of Drilling Contractors (IADC) defines managed pressure drilling (MPD) as an adaptive drilling process used to precisely control the annular pressure profile throughout a wellbore. The objectives are to ascertain the down hole pressure environment limits and to manage the annular hydraulic pressure profile accordingly. MPD systems comprise a closed pressure system for providing automatic control of the backpressure within a wellbore during a drilling process [or other drilling and completion operations].
The bulk modulus of a substance measures the substance's resistance to uniform compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. In conventional MPD systems, nominal values of bulk modulus are used, which do not account for cuttings, temperature variations, and other real-world effects. The effective bulk modulus describes the compressibility of the fluids in the annulus. The fluid compressibility can vary by at least a factor of four in a conventional drilling operation. As the fluid compressibility changes so to do the dynamics of the drilling process. This has implications for the optimal MPD controller settings such as gain. The bulk modulus is affected by several factors which make it difficult to estimate, e.g. gas in the drilling mud, expansion of the casing and wellbore, and temperature gradients all contribute to the overall effective bulk modulus of the annulus.
An MPD system is described in GB2473672 B. The following patent documents are also concerned with MPD systems; WO2008016717, US2005269134, US2005092523, US2005096848 and U.S. Pat. No. 7,044,237.